.TH std::freopen 3 "2024.06.10" "http://cppreference.com" "C++ Standard Libary"
.SH NAME
std::freopen \- std::freopen

.SH Synopsis
   Defined in header <cstdio>
   std::FILE* freopen( const char* filename, const char* mode, std::FILE* stream );

   First, attempts to close the file associated with stream, ignoring any errors. Then,
   if filename is not null, attempts to open the file specified by filename using mode
   as if by std::fopen, and associates that file with the file stream pointed to by
   stream. If filename is a null pointer, then the function attempts to reopen the file
   that is already associated with stream (it is implementation defined which mode
   changes are allowed in this case).

.SH Parameters

   filename - file name to associate the file stream to
   mode     - null-terminated character string determining new file access mode
   stream   - the file stream to modify

   File access flags

    File access        Meaning         Explanation      Action if file  Action if file
    mode string                                         already exists  does not exist
   "r"            read              Open a file for    read from start  return NULL and
                                    reading                             set error
   "w"            write             Create a file for  destroy contents create new
                                    writing
   "a"            append            Append to a file   write to end     create new
   "r+"           read extended     Open a file for    read from start  return NULL and
                                    read/write                          set error
   "w+"           write extended    Create a file for  destroy contents create new
                                    read/write
   "a+"           append extended   Open a file for    write to end     create new
                                    read/write
   File access mode flag "b" can optionally be specified to open a file in binary mode.
   This flag has no effect on POSIX systems, but on Windows, for example, it disables
   special handling of '\\n' and '\\x1A'.
   On the append file access modes, data is written to the end of the file regardless
   of the current position of the file position indicator.
   File access mode flag "x" can optionally be appended to "w" or "w+" specifiers. This
   flag forces the function to fail if the file exists, instead of overwriting it.
   \fI(C++17)\fP
   The behavior is undefined if the mode is not one of the strings listed above. Some
   implementations define additional supported modes (e.g. Windows).

.SH Return value

   stream on success, a null pointer on failure.

.SH Notes

   std::freopen is the only way to change the narrow/wide orientation of a stream once
   it has been established by an I/O operation or by std::fwide.

   Microsoft CRT version of std::freopen does not support any mode changes when
   filename is a null pointer and treats this as an error (see documentation). A
   possible workaround is the non-standard function _setmode().

.SH Example

   The following code redirects stdout to a file.


// Run this code

 #include <cstdio>

 int main()
 {
     std::printf("stdout is printed to console\\n");
     if (std::freopen("redir.txt", "w", stdout))
     {
         std::printf("stdout is redirected to a file\\n"); // this is written to redir.txt
         std::fclose(stdout);
     }
 }

.SH Output:

 stdout is printed to console

.SH See also

   fopen  opens a file
          \fI(function)\fP
   fclose closes a file
          \fI(function)\fP
   C documentation for
   freopen
